A motor-vehicle headrest is typically supported on a pair of support tubes whose upper ends are fixed in the headrest and whose lower ends are slidable in respective vertical sleeves set in a seat back. At least one of the tubes is formed with an array of outwardly open notches into which can fit an element of a latch provided in the seat back. The notches typically are formed with a downwardly facing latch face that lies in a plane generally perpendicular to the axis of the tube and a cam or ramp face that extends radially outward from the base of the latch face to the cylindrical outer surface of the tube, forming a small acute angle with the tube axis. A small intermediate surface lying in a plane parallel to the axis may extend from the inner edge of the latch face to the inner edge of the cam or ramp face. The latch element that fits in these notches to prevent downward displacement of the headrest is typically spring-biased against the tube so that the tube can be moved up without radially retracting the latch element but can only be moved down when the element is retracted radially outward away from the tube. In this way the headrest can be raised to a desired position without actuating the latch but can only be moved back down when the latch is actuated.
The use of a tube rather than a solid rod has the advantage that weight can be reduced. Furthermore, the material, being displaced during the formation of the notch in a solid rod would project beyond the outer contour of the support rod and must therefore be removed utilizing an additional manufacturing step, for example, by machining. In a support tube the material may be displaced into the interior of the tube such that no subsequent machining is necessary.
The formation of the notches is carried out by a die that is pressed against the outside of the tube and whose end corresponds to the outside surfaces of the notch and a mandrel that fits inside the tube and whose surface is formed with recesses corresponding to the inside surface of the tube at the notches to be formed. The die has a plane surface which forms the intermediate surface at the base of the latch face and a planar surface adjacent that forms the ramp. The formation of tears—particularly, if the notch depth is greater than the wall thickness of the tube—often occurs in the corner between the notch bottom and the latch face.
Depending on the circumference of the tear formation the quality of the support rod has been previously reduced, or has resulted in rejection of the finished support tube, which in turn resulted in high costs depending on the support rod material and manufacturing steps.